1. Technical Field
This disclosure relates to a thermosensitive recording material that can be used in a wide variety of fields, for example, in printers for computer output and calculators, recorders for medical instruments, low-speed and high-speed facsimiles, automatic ticket-vending machines, thermosensitive photography, handy terminals, and labels for the POS system.
2. Description of the Related Art
Various types of recording materials have been proposed in which a thermosensitive recording layer that contains as main components a colorless or light-colored leuco dye and a developer that produces color upon contact with the dye is provided on a support such as paper, synthetic paper and plastic film, so that developing reaction between the leuco dye and developer effected by application of heat or pressure are utilized. These types of thermosensitive recording materials require no troublesome treatments such as developing and fixing, offering such advantages as shorter recording time with a relatively simple device, low noise level, and low costs. These advantages have enabled them to be available not only for copying of books and documents, but also as recording materials for use in a variety of fields including computers, facsimiles, ticket machines, label printers, recorders, and handy terminals.
What is demanded for thermosensitive recording materials is quick, highly dense/sensitive developing, with high robustness in the developed image and background.
In regard to attempts to achieve increased sensitivity, methods have been proposed in which the thermal conductivity of a support is appropriately defined (Japanese Patent Application Laid-Open (JP-A) No. 55-164192) and in which an intermediate layer containing various types of hollow microparticles is formed on a support (JP-A Nos. 59-5093 and 59-225987). However, in these cases, it is difficult to form a uniform intermediate layer and the surface readily becomes uneven, leading to poor resolution (dot reproducibility) in the formed image. Furthermore, other methods have been proposed in which styrene acrylic resin and polystyrene resin are used as the partition materials of the above-described hollow microparticles (JP-A No. 63-281886) and in which an intermediate layer is formed, the main components of which are non-foamed hollow microparticles with a hollow ratio of 30% or more (JP-A No. 02-214688). However, even in these cases, adequate heat-insulating effects cannot be obtained since the hollow ratio is low, so that the highly sensitive thermosensitive recording material current being sought cannot be obtained.
Also, a method has been proposed in which an intermediate layer containing hollow particles with a hollow ratio of 80% or more is formed (JP-A No.05-573). This method can produce a thermosensitive recording material with higher sensitivity. However, when a thermosensitive color-developing layer is formed on the intermediate layer with, for example, a wire bar and a blade, considerable friction occurs between the coating head thereof and the intermediate layer, which degrades transferability. In addition, debris is likely to be formed due to friction, resulting in that high-speed coating cannot be performed. As described above, satisfactory results cannot be obtained with this method.
With regard to high-speed coating associated with recent demands for increased productivity, a curtain coating method has received attention for its advantages including significant reduction in expenditure involved in drying equipment and energy, which are achieved by increased coating speed and simultaneous multilayer coating. JP-A No. 2003-182229 discloses producing a thermosensitive recording layer by a curtain coating method in order to obtain a thermosensitive recording material which is excellent in sensitivity, image quality and head-matching property.
The curtain coating is a contour coating and thus, is suitably used for coating of a support with high smoothness (e.g., a film). However, when the support is a paper sheet with uneven surface profile, a smooth coated surface cannot be obtained, leading to uneven printing and/or degradation in glossiness of the printed product.
Furthermore, in recent years, such thermosensitive recording materials have come to be abundantly used in fields where fidelity of recorded images is deemed critical, such as labels and receipts. Accordingly, thermosensitive recording materials are in demand that have high resistance against water and acidic substances in foods, and oils and fats and plasticizers in organic polymer materials used in packages. Meanwhile, thermosensitive recording materials are used under various environmental conditions. In particular, when used under high-temperature, high-humidity conditions, the thermosensitive recording materials may involve sticking by which normal printing is inhibited and thus, are required to have excellent head-matching property.
There have been attempts to overcome the aforementioned problems for instance by providing a protective layer on the thermosensitive recording layer. In particular, it has been proposed that polyvinyl alcohols or modified polyvinyl alcohols be used as the resin for a protective layer, and that these polyvinyl alcohols and a waterproofing agent be used together as the protective layer.
For example, JP-A No. 08-151412 discloses using a hydrazine compound and a diacetone group-containing polyvinyl alcohol, but when they are used in a protective layer of the thermosensitive recording material, the reaction for waterproofness is promoted in their coating liquids followed by unwanted increase in viscosity over time. Also, JP-A No. 10-291367 discloses using an acetoacetyl-modified polyvinyl alcohol in a thermosensitive recording layer or protective layer, and using a ketone resin as a crosslinking agent. Further, JP-A No. 11-314458 discloses using an acetoacetyl-modified polyvinyl alcohol in a protective layer and using a hydrazine compound serving as a crosslinking agent in a thermosensitive recording layer. In both cases, the formed protective layer is poor in waterproofness and head-matching property. In addition, JP-A No. 11-314457 proposes that a diacetone-modified polyvinyl alcohol be used in the resin of a protective layer and that a hydrazine compound be contained in the thermosensitive color-developing layer, but the problems arise that the waterproofness of the protective layer is insufficient, the viscosity of the coating liquid on the thermosensitive color-developing layer increases and developing of the thermosensitive color-developing layer is inhibited by the hydrazide compound. Furthermore, in JP-A No. 10-87936, a waterproofing method is proposed that uses a water-soluble amine, a hydrazide compound and a polyvinyl alcohol copolymer containing diacetone acryl amide as a monomer. However, when they are used in a protective layer of the thermosensitive recording material, the amine undesirably affects the thermosensitive color-developing layer to cause coloring in the background, pH control by the amine becomes difficult and, depending on the amount of amine added, viscosity increases conversely.
Regarding increase in viscosity, JP-A No. 2002-283717 attempts to solve this problem by using a hydrazide compound as a crosslinking agent for a polyvinyl alcohol having a reactive carbonyl group, and also by incorporating a basic filler.
When thermosensitive recording materials that use a hydrazide compound and polyvinyl alcohol containing a reactive carbonyl group are used, however, an image printed with aqueous ink for flexography is easily peeled off by external force after long-time exposure to water.
Meanwhile, the curtain coating method has received attention for its advantages including significant reduction in expenditure involved in drying equipment and energy, which are achieved by increased coating speed and simultaneous multilayer coating associated with recent demands for increased productivity. JP-A No. 2003-182229 discloses producing a thermosensitive recording layer and a protective layer by a curtain coating method similar to the present invention, in order to obtain a thermosensitive recording material which is excellent in sensitivity, image quality and head-matching property.
However, this patent literature remains silent with respect to a thermosensitive recording material which offers a printed image excellent in waterproofness and head-matching property and which can be produced through high-speed coating. Needless to say, this neither describes nor suggests that a protective layer having a two-layered structure is formed, and that a maleic acid-modified polyvinyl alcohol is contained in the second protective layer.
As described above, none of them has provided a thermosensitive recording material which can be produced through high-speed coating and which has high sensitivity and excellent storage stability.